This invention relates to methods of manufacturing semiconductor electronic devices, such as MOS integrated circuits (ICs), incorporating layers of dielectric material and also incorporating at least one layer of metal.
The oxide layer of an MOS structure must be of relatively high purity. Mobile ions, for example sodium ions, are known to occur in silicon dioxide layers formed by conventional methods. These ions are detrimental to the performance of MOS devices. In silicon field-effect transitors, for example, mobile ions in the gate oxide layer cause shifts in the operating voltage of the device. One conventional, non-destructive method that has been developed for measuring the mobile-ion concenttration is silicon dioxide layers is the Triangular-Voltage-Sweep (TVS) method.
Typically, the TVS method is applied to selected test portions of a wafer to be processed. Each of the test portions is configured as an MOS capacitor, having a silicon substrate, a dielectric layer, typically of silicon dioxide, in contact with the substrate, and a metal contact, typically an aluminum contact, formed in contact with the dielectric layer on the side opposite to the substrate. Significantly, two interfaces are defined by these three layers: an interface between the silicon substrate and the dielectric layer, and a second interface between the dielectric layer and the metal contact.
The TVS method is described, for example, in M. Kuhn and D. J. Silversmith, "Ionic Contamination and Transport of Mobile Ions in MOS Structures," J. Electrochem. Soc., Vol. 118, Pg. 966 (1971) , and in N. J. Chou, "Application of Triangular Voltage Sweep Method to Mobile Charge Studies in MOS Structures," J. Electrochem. Soc., Vol. 118, Pg. 603 (1971). Briefly, during testing of an MOS capacitor, the capacitor is maintained at an elevated temperature, typically 100.degree.-400.degree. C., while a triangular voltage sweep is applied across the capacitor. That is, the applied voltage is varied continuously and linearly with time from a negative extreme of, e.g., -10 V to an equal but opposite positive extreme of, e.g., 10 V, and then returned to the initial voltage in the same manner. (The positive and negative voltage extremes must be sufficient to produce an average electric field of approximately 1 MV/cm in the dielectric layer being tested. Thus the actual voltages required are determined by the thickness of the dielectric layer.) The sweep rate is relatively slow, typically about 5-100 mV per second. (For present purposes, the applied voltage if negative is the silicon substrate is negative relative to the dielectric layer.) As the applied voltage is varied, the displacement current (i.e., the time rate of change of the charge induced at the silicon-dielectric interface by the applied voltage) is continuously monitored. The displacement current has, in addition to an electronic component, a component due to the motion of mobile ionic impurites, for example, sodium ions. A graph, here called a characteristic curve, is readily constructed in which the vertical axis represents the displacement current and the horizontal axis represents the sweep voltage (or sweep time, which is typically proportional to the sweep voltage). In many dielectrics, the ionic component due to alkali-metal ions appears in the characteristic curve as a well-defined peak appearing near mid-sweep, i.e., near zero applied volts. The area under the peak is proportional to the concentration of mobile ionic impurities.
Additionally, if defects or a conduction path are present in the dielectric, a leakage current may appear. The presence of a leakage current causes the characteristic curve to have a non-zero slope. However, this effect is readily prevented by growing a blocking layer of a highly insulating dielectric, such as thermally grown silicon dioxide, between the silicon substrate and the dielectric layer to be tested.
From the characteristic curve, it is possible to infer quantitative information about the mobile-ionic-charge distribution. However, such inferences are drawn under two assumptions. The first assumption is that the total number of ions is constant during the voltage sweep. The second assumption is that the ionic-charge distribution is in quasi-static equilibrium; that is, the assumption that at every moment, the ionic-charge distribution is in equilibrium with the instantaneous value of the applied voltage, in conformance with Poisson's equation.
If quasi-static equilibrium applies, then the ionic component, I.sub.ionic, of the displacement current is expressed by ##EQU1## where x.sub.0 is the thickness of the dielectric, .alpha. is the (linear) sweep rate, .rho. is the ionic-charge distribution, and the integral represents a weighted average areal-charge density. In that case, the area under the TVS peak is equal to .alpha. times the difference between the weighted averages at the respective endpoints of the voltage sweep. In particular, if essentially all of the mobile charge is acumulated at one interface by applying a constant voltage of at least the peak sweep amplitude to the sample prior to the sweep, the area of the resulting peak will be proportional to the total areal-charge density due to mobile ions.
However, the assumption of quasi-static equilibrium will only apply if the applied voltage is varied relatively slowly. Kuhn and Silversmith, for example, estimate that at temperatures above 150.degree. C., equilibrium may be expected to apply at sweep rates less than 100 mV/sec. As a consequence, the conventional TVS method is useful for obtaining quantitative information about ionic-charge distributions only at sweep rates less than, typically, a few tenths of a volt per second.